Working with soil chemists, we've gone over the list of potential soil extractants used in various places for extracting various materials. There are at least a minimum of 100 different ways to extract different nutrients from soil.
Part of the problem in comparing one lab's results with another is that they use different extractants. Different extractant procedures result in pulling different amounts of exchangable nutrients out of soil.
It makes a difference if you add water first, and then add the acid, or put acid in first, and then add the water. You'll get different answers. So, what does your lab do?
We need to know how to interpret different soil chemical extracting procedures so we know how much of the total exchangeable pool is actually being pulled out of the soil with any particular soil extracting agent.
This of course is relative to the pH of soil to begin with. Different extractants remove exchangeable nutrients at different levels of efficiency as pH changes.
So why not put each soil into the same pH material to start out with? Because you just changed what is actually available in real-world conditions by doing that.
What we want to know from chemical testing is: How much nutrient is available for the plant to take up?
Do our current testing methods give us that information? Probably not.
I like Arden Anderson's explanation of different extracting methods. If you use really STRONG extracting agents, you get an idea of how much food you have in the house. That includes the freezer, refrigerator and on-the-table resources. In soil terms, that's total exchangeable nutrients.
The weaker extracting agents tell you how much is in the refrigerator and on the table. In soil terms, if you will, these are the easily exchangeable nutrients.
The Reams extracting agents, or the weakest extracting agents, assess
what's on the table. In soil terms, these are the water soluble nutrients. The nutrients that have been pulled from exchangeable sites into the soil solution.
Do any of these predict what the plant WILL take up? No.
Just like a person, just because the food is in the freezer, the refrigerator, or even prepared and on the table, does not mean we will actually eat it. Just because nutrients are in water solution does not necessarily mean the plant can or will take it up.
So, it leaves us still with the need to understand how to measure, in a truly predictive manner, what the plant will take up.
Soil biology? Um, but which part of soil biology? How do know which set of organisms is most important in making any particular nutrient available to plants?
Well, we have some data that we're trying to put together, and lots more testing to figure this out.
But, we can rank the different extracting agents and methods so we get a relative idea of which methods should be getting more or less of the total exchangeable pool, under which pH conditions, with different clay compositions.
And then -
hold on, this isn't all the chemistry you need to understand -
add in an understanding of the TOTAL EXTRACTABLE pool of nutrients.
What is the total amount of nutrient, that if you have a full foodweb present and performing their functions, will all be moved into the water soluble pool, and presumably, with the right biology present, become plant available?
In the assessments of soil chemistry underway with researchers in Australia, at Southern Cross University, there is no soil in Australia that lacks phosphorus. Or boron. Or silica. Or any nutrient, except perhaps nitrogen. If agricultural practices have really beaten on the soil, sulfur may be low as well. But no where near zero, and certainly, not limiting for plant growth for several crops at least.
The nutrients are in our soils. Only when soils have been hammered by poor ag practices, or by severe disturbances, should significant additions of nutrients be required. Otherwise, all you need is the biology to move the nutrients that are present, but sequestered and tied up in soil, into soluble pools for plant to take-up.
So, you can add the huge amounts of inorganic fertilizers recommended every year, forever, and destroy water quality at the same time, or add the biology needed, plus the foods to feed them, and exit the toxic chemcials from your farm shelves. You add back in the nutrients needed for plants in the foods for the microbes. You only have to replace what the plant took out. Which, based on testing the TOTAL NUTRIENT pool in compost, is only perhaps a half ton of well-made, aerobic compost, where N, S, or P have NOT been lost as a result of anaerobic conditions which develop when the organic matter is incorrectly "composted".
In the world of research, we need to figure out exactly which organisms are needed for each nutrient-solubilizing step, in all the conditions your soil is likely to need nutrient-solubilizing to occur, and make sure those organisms are present, and have food. Most of the time, plants will add the needed foods. As long as you keep plant cover on your soils, the organisms will keep working for you.
Not too much though. Balance is important.
Well, lots more to go through to get the complete story, but I shouldn't try to cover everything in one fell swoop.
Elaine R. Ingham
Soil Foodweb Inc., Corvallis, Oregon
Soil Foodweb Inc., Port Jefferson, New York
Soil Foodweb Institute, Lismore Australia
Soil Foodweb Institute Cambridge, New Zealand
Laboratorios de Soil Foodweb, Culiacan, Mexico
Soil Foodweb Inc., Jerome, Idaho
Soil Foodweb Inc., South Africa
To unsubscribe from SANET-MG:
1- Visit http://lists.sare.org/archives/sanet-mg.html and unsubscribe by typing in your e-mail address or;
2- Send a message to mailto:email@example.com from the address subscribed to the list. Type "unsubscribe sanet-mg" in the body of the message.
Visit the SANET-MG archives at: http://lists.sare.org/archives/sanet-mg.html